Image guided radiotherapy (IGRT) has become the state of the art in radiation treatment. IGRT may utilize two-dimensional (2D) projection images or three-dimensional (3D) cone beam computed tomography (CT) images that are acquired prior to treatment. These images are compared to a set of pre-treatment images to ensure the patient has been set up accurately and consistently each treatment session. While CT, for example reconstructs in a standard cartesian geometry, the geometry of the radiation treatment beam originates from a divergent source. Beam's eye view (BEV) projection is commonly used to visualize patient anatomy to determine exactly what tissue will be irradiated from the divergent treatment beam.
In BEV projection, the 3D image dataset captured by CT, referred to the portal image, is registered to a reconstructed BEV image. The BEV image is reconstructed by ray tracing through the 3D CT dataset of the portal image from a virtual source position aligned with the location of the target of the radiation treatment beam source. Since the BEV image represents the path of the divergent radiation treatment beam, target coverage and critical structure avoidance can be accurately determined. This makes the BEV image an ideal image to use for real-time IGRT where the image must be acquired and analyzed in real time to reposition the beam to conform to the target volume while avoiding any surrounding radiation sensitive organs. However, processing the CT dataset in real time to generate a BEV image is computationally intensive and cannot be completed in real-time.
Magnetic resonance (MR) guided radiotherapy treatment systems integrate magnetic resonance imaging (MRI) devices with radiotherapy treatment systems. For example, U.S. Pat. No. 8,983,573, incorporated by reference in its entirety herein, is directed to a radiation therapy system that comprises a combined MRI apparatus and a linear accelerator capable of generating a beam of radiation.
In MRI, a signal from a slab of selected tissue gives rises to a two-dimensional slice that is integrated in a direction perpendicular to the slab. If the slab is oriented horizontally, each pixel in the 2D image is generated by summing the signal together along a vertical line. A radiation beam originating from a radiation source (a point source location) diverges from the source and fans out over the target. Thus, targeting the radiation source based on this conventionally obtained image may lead to a decreased radiation dose at the target than what was planned and/or unnecessary dose to tissue surrounding the target. For example, a 5 cm thick image slice may result in up to 4 mm of targeting error, 15 cm from the central beam axis, assuming a 100 cm distance from the radiation source. This effect will increase further as slice thickness increases.
While it may be possible to process the image data collected in MRI by ray tracing to produce an BEV image, similar to the methodology described above for CT, this processing is performed after the data is captured and therefore not available in real-time. Furthermore, the 3D image data collected in MRI requires significantly more data acquisition in general than does the image data collected in CT. As a result, significant time is required to produce a BEV image.
Accordingly, it is an object of the present invention to obviate or mitigate at least one of the above-noted disadvantages.